My computer has recently been making a strange noise when I switch it on. As I hit the button, for about two seconds there's a noise like a moped engine revving &revving, and then it goes away, and the computer gets on with booting up. At first, I thought it was the fan in the power supply, so I put a new PS in. The noise went away for about a fortnight, but now it's back, so whatever's causing the noise, it's unlikely to be the PS. I'm worried it could be the hard drive getting tired. Any ideas?